This invention relates to an objective lens for an optical pickup, employed for irradiating the laser light as a light beam on a signal recording surface of a disc-shaped recording medium, that is an optical disc, the optical pickup employing this objective lens, and to an optical disc device.
An optical disc is now in use as an information recording medium, specifically, a recording medium which allows to record and/or reproduce the information without a recording and/or reproducing head contacting with the recording medium, and in which it is attempted to increase the recording density. The information recorded on this optical disc may be read and reproduced by irradiating the laser light radiated from the optical pickup as a recording and/or reproducing head to a signal recording layer of the optical disc, and by detecting the laser light reflected by the signal recording layer by a photodetector provided in the optical pickup.
Among the optical discs capable of recording and/or reproducing the information, there is an optical disc that employs a phase change recording layer which is changed from a crystallized state to a non-crystallized state and vice versa on irradiation of, for example, the laser light thereon. The information recording for this sort of the optical disc is by irradiating the signal recording layer with the laser light modulated in accordance with the information signals for recording.
As is typical of an optical disc that employs the phase change recording layer to allow for information recording and/or reproduction, there is a DVD (Digital Video Disc or Digital Versatile Disc). The DVD is improved appreciably in recording density as compared to CD which is the optical disc now finding widespread use. The track pitch of the recording track of the DVD is 0.74 m, in comparison with 1.6 m in the case of the CD, thus achieving high recording density.
For irradiating the laser light on the optical disc to allow high density information recording, it is necessary to form a beam spot of yet a smaller size on a signal recording surface of the optical disc. Since the spot diameter of the laser light is proportionate to the design wavelength of the laser light and inversely proportionate to the numerical aperture (NA) of the objective lens converging the laser light, it is necessary to increase the numerical aperture of the objective lens and to diminish the wavelength of the laser light if high recording density of the laser light is to be achieved.
For further improving the recording density of the optical disc, such an optical pickup has been proposed in which the wavelength of the laser light used for recording and/or reproducing the information is 405 nm and in which the numerical aperture (NA) of the objective lens is 0.85 and approximately 1.5 when ultra-resolution is not used, when ultra-resolution is used, and when the lens used is a so-called solid immersion lens, abbreviated to SIL, respectively.
As a technique for increasing the numerical aperture (NA) of the objective lens for achieving high recording density of the optical disc, a double lens type objective lens, typified by SIL, has been proposed. In the SIL, exploiting light bleed-out in an objective lens with the numerical aperture (NA) not less than 1, the working distance (WD), that is a distance between the foremost part of the objective lens and the signal recording layer of the laser light, as a light collecting point, is on the order of tens of nm.
As an objective lens, in which the numerical aperture (NA) is increased without resorting to SIL, the objective lens shown in Japanese Laying-Open Patent Publication H-10-104507, and the objective lens shown in FIG. 1 hereof, have so far been proposed. In these objective lenses, shown in the above Patent Publication or in FIG. 1 hereof, a double-lens structure, similar to that used in the SIL, is used. With this lens structure, it is targeted to scatter the power between the two lenses to relieve the load of the refractive power imposed on the respective surfaces. This lens structure is meritorious in the perspective of lens machining because the radius of curvature of each lens surface may be moderated, and also because the aspherical coefficient may be diminished.
However, with the objective lens 200, shown in FIG. 1, a first lens 201 and a second lens 202 are arranged along the direction of an optical axis P to distribute the refractive power between the first and second lenses 201, 202. Thus, the principal point is intruded into the inside of the objective lens 200, with the result that the WD up to a signal recording surface 210a of the optical disc 105 is diminished. Meanwhile, the WD of the objective lens 200 shown in FIG. 1 is 0.15 mm.
Although the numerical aperture (NA) of the objective lens 200 as shown in FIG. 1, designed as described above, may be increased, the WD is small, so that, when the objective lens is built into the optical pickup for recording and/or reproducing the optical disc, the objective lens collides against the optical disc. Specifically, such collision of the objective lens against the optical disc unavoidably occurs due to deviation of the surface of the rotationally driven optical disc from planarity or to movement of the objective lens along the optical axis in the course of focussing control of the laser light.
With the objective lens, shown in FIG. 1, the beam diameter of the laser light just emanated from the objective lens is extremely small, so that, even if small-sized dust and dirt become attached to the laser light radiating surface of the objective lens or to the laser light incident surface of the optical disc, the laser light cannot be converged with a sufficient light volume on the signal recording surface of the optical disc.
In the case of the objective lens, combined from two lenses, it is required to maintain a highly accurate distance between the forward and rear lenses and to assemble the lenses without producing offset or tilt to render production and assembling extremely difficult. With the objective lens 200, shown in FIG. 1, it is required to suppress distance errors between the first and second lenses 201, 202 to 5 m or less, as well as to suppress the tilt between the lenses 201, 202 to 1 mrad or less.
In order to solve the problem proper to the double type objective lens unit, a demand is raised for a single type lens unit, as an objective lens for an optical pickup, in which it is possible to increase the WD and in which there is no necessity for combining plural lenses.
With the single type objective lens unit, so far proposed or put to practical use, if the numerical aperture (NA) is increased, the first surface of the lens, as a laser light incident surface, has a depth (sag) which is increased as compared to the basic curvature, thus presenting difficulties in machining employing a metal die device for molding.
In view of the above-depicted status of the art, it is an object of the present invention to provide a novel objective lens usable with advantage with an optical pickup, used for recording and/or reproducing the information for an optical recording medium, the recording density of which has been raised by narrowing the track pitch of the recording track. It is a more specific object of the present invention to provide an objective lens for an optical pickup that is able to converge the laser light to close to the diffraction limit.
It is also a specific object of the present invention to provide an optical pickup employing an objective lens capable of converging the laser light to close to the diffraction limit, and an optical disc device employing this optical pickup.
It is a further specific object of the present invention to provide an objective lens for an optical pickup which may be readily machined using a metal die for molding.
For accomplishing the above object, the present invention provides an objective lens for an optical pickup, made up by a single lens of a meniscus structure including, looking sequentially from a light source side, a first surface formed by an aspherical surface, having a positive refractive power, and a second surface, formed by an aspherical surface, having a negative refractive power, with the numerical aperture (NA) of the lens being not less than 0.8, wherein the aspherical shape Zi of the aspherical surfaces of the first and second surfaces is given by the equation:       Z    i    =                              r          i                      -            1                          ⁢                  xe2x80x83                ⁢                  h          2                            1        +                              1            -                                          (                                  1                  +                                      k                    i                                                  )                            ⁢                              xe2x80x83                            ⁢                              r                i                                  -                  2                                            ⁢                              xe2x80x83                            ⁢                              h                2                                                          +                  A        i            ⁢              xe2x80x83            ⁢              h        4              +                  B        i            ⁢              xe2x80x83            ⁢              h        6              +                  C        i            ⁢              xe2x80x83            ⁢              h        8              +                  D        i            ⁢              xe2x80x83            ⁢              h        10              +                  E        i            ⁢              xe2x80x83            ⁢              h        12              +                  F        i            ⁢              xe2x80x83            ⁢              h        14              +                  G        i            ⁢              xe2x80x83            ⁢              h        16              +                  H        i            ⁢              xe2x80x83            ⁢              h        18              +                  J        i            ⁢              xe2x80x83            ⁢              h        20            
with the height from the optical axis being h, where i is the surface number, as counted from the light source side, r is the radius of curvature of the aspherical surface, k is the cone constant, with k less than 0, and A to J denote aspherical coefficients.
Moreover, the following condition:
(r1/r2)xe2x89xa6(2N2xe2x88x92Nxe2x88x924)/{N(2N+1)}
is met, where r1 is the radius of curvature of the first surface, r2 is the radius of curvature of the second surface and N is a refractive index of the vitreous material.
In addition, with an objective lens for an optical pickup according to the present invention, the following condition:
r1 less than fxc2x7NA
is met, where r1 is the radius of curvature of the first surface, f is the focal length and the NA is the numerical aperture.
The present invention also provides an objective lens for an optical pickup, made up by a single lens of a meniscus structure including, looking sequentially from a light source side, a first surface formed by an aspherical surface, having a positive refractive power, and a second surface, formed by an aspherical surface, having a negative refractive power, with the numerical aperture (NA) of the lens being not less than 0.8, wherein the aspherical shape Zi of the aspherical surfaces of the first and second surfaces is given by the equation:       Z    i    =                              r          i                      -            1                          ⁢                  xe2x80x83                ⁢                  h          2                            1        +                              1            -                                          (                                  1                  +                                      k                    i                                                  )                            ⁢                              xe2x80x83                            ⁢                              r                i                                  -                  2                                            ⁢                              xe2x80x83                            ⁢                              h                2                                                          +                  A        i            ⁢              xe2x80x83            ⁢              h        4              +                  B        i            ⁢              xe2x80x83            ⁢              h        6              +                  C        i            ⁢              xe2x80x83            ⁢              h        8              +                  D        i            ⁢              xe2x80x83            ⁢              h        10              +                  E        i            ⁢              xe2x80x83            ⁢              h        12              +                  F        i            ⁢              xe2x80x83            ⁢              h        14              +                  G        i            ⁢              xe2x80x83            ⁢              h        16              +                  H        i            ⁢              xe2x80x83            ⁢              h        18              +                  J        i            ⁢              xe2x80x83            ⁢              h        20            
with the height from the optical axis being h, where i is the surface number, as counted from the light source side, r is the radius of curvature of the aspherical surface, k is the cone constant, with k less than 0, and A to J denote aspherical coefficients.
Moreover, the following condition:
r1 less than fxc2x7NA
is also met, where r1 is the radius of curvature of the first surface, f is the focal length and the NA is the numerical aperture.
The objective lens for the optical pickup according to the present invention is formed of a vitreous material having a refractive index of not less than 1.7.
The present invention also provides an optical pickup including a light source radiating a laser light beam, a beam splitter, an objective lens for converging the laser light on a signal recording surface of a disc-shaped recording medium, and a light receiving element for receiving the laser light reflected back from the signal recording surface of the disc-shaped recording medium. The objective lens for this optical pickup includes, looking sequentially from a light source side, a first surface formed by an aspherical surface, having a positive refractive power, and a second surface, formed by an aspherical surface, having a negative refractive power, with the numerical aperture (NA) of the lens being not less than 0.8. The aspherical shape Zi of the aspherical surfaces of the first and second surfaces is given by the equation:       Z    i    =                              r          i                      -            1                          ⁢                  xe2x80x83                ⁢                  h          2                            1        +                              1            -                                          (                                  1                  +                                      k                    i                                                  )                            ⁢                              xe2x80x83                            ⁢                              r                i                                  -                  2                                            ⁢                              xe2x80x83                            ⁢                              h                2                                                          +                  A        i            ⁢              xe2x80x83            ⁢              h        4              +                  B        i            ⁢              xe2x80x83            ⁢              h        6              +                  C        i            ⁢              xe2x80x83            ⁢              h        8              +                  D        i            ⁢              xe2x80x83            ⁢              h        10              +                  E        i            ⁢              xe2x80x83            ⁢              h        12              +                  F        i            ⁢              xe2x80x83            ⁢              h        14              +                  G        i            ⁢              xe2x80x83            ⁢              h        16              +                  H        i            ⁢              xe2x80x83            ⁢              h        18              +                  J        i            ⁢              xe2x80x83            ⁢              h        20            
with the height from the optical axis being h, where i is the surface number, as counted from the light source side, r is the radius of curvature of the aspherical surface, k is the cone constant, with k less than 0, and A to J denote aspherical coefficients.
In addition, the following condition:
(r1/r2)xe2x89xa6(2N2xe2x88x92Nxe2x88x924)/{N(2N+1)}
is met, where r1 is the radius of curvature of the first surface, r2 is the radius of curvature of the second surface and N is a refractive index of the vitreous material.
With the objective pickup the following condition:
r1 less than fxc2x7NA
is also met, where r1 is the radius of curvature of the first surface, f is the focal length and the NA is the numerical aperture.
The present invention also provides an optical pickup including a light source radiating a laser light beam, a beam splitter, an objective lens for converging the laser light on a signal recording surface of a disc-shaped recording medium, and a light receiving element for receiving the laser light reflected back from the signal recording surface of the disc-shaped recording medium. The objective lens for the optical pickup includes, looking sequentially from a light source side, a first surface formed by an aspherical surface, having a positive refractive power, and a second surface, formed by an aspherical surface, having a negative refractive power, with the numerical aperture (NA) of the lens being not less than 0.8. The aspherical shape of the aspherical surfaces of the first and second surfaces is given by the equation:       Z    i    =                              r          i                      -            1                          ⁢                  xe2x80x83                ⁢                  h          2                            1        +                              1            -                                          (                                  1                  +                                      k                    i                                                  )                            ⁢                              xe2x80x83                            ⁢                              r                i                                  -                  2                                            ⁢                              xe2x80x83                            ⁢                              h                2                                                          +                  A        i            ⁢              xe2x80x83            ⁢              h        4              +                  B        i            ⁢              xe2x80x83            ⁢              h        6              +                  C        i            ⁢              xe2x80x83            ⁢              h        8              +                  D        i            ⁢              xe2x80x83            ⁢              h        10              +                  E        i            ⁢              xe2x80x83            ⁢              h        12              +                  F        i            ⁢              xe2x80x83            ⁢              h        14              +                  G        i            ⁢              xe2x80x83            ⁢              h        16              +                  H        i            ⁢              xe2x80x83            ⁢              h        18              +                  J        i            ⁢              xe2x80x83            ⁢              h        20            
with the height from the optical axis being h, where i is the surface number, as counted from the light source side, r is the radius of curvature of the aspherical surface, k is the cone constant, with k less than 0, and A to J denote aspherical coefficients. In addition, the following condition:
r1 less than fxc2x7NA
is met, where r1 is the radius of curvature of the first surface, f is the focal length and the NA is the numerical aperture.